Field of the Invention
The present invention relates to cutting, more particularly, relates to a cutting method and a cutting apparatus which are capable of accurate cutting with a low incidence of chatter vibration and the like in a work.
Description of the Related Art
Cutting is generally performed by the structure shown in the illustration of FIG. 11. In FIG. 11, reference numeral 35 indicates a workpiece (work) to be cut, reference numeral 36 indicates a tool post, reference numeral 37 indicates a cutting tool, reference numeral 38 indicates a motor, reference numeral M1 indicates an axis of the work 35, and reference numeral M5 indicates a tool axis. As shown in FIG. 11, the tool post 36 has the cutting tool 37 so that the axis M1 of the work 35 and the tool axis M5 cross each other. The tool post 36 is translatable in an X-axis direction by the motor 38. Outer-diameter cutting is performed by cutting the work 35 in the axis direction of the work 35 with the cutting tool from the X-axis direction by the motor 38, and then feeding the cutting tool 37 in the work axis-M1 direction while rotating the work 35.
However, performing heavy cutting by this cutting method generates chatter vibration when the work 35 has low dynamic rigidity, thereby causing problems in some cases, such as reducing a finishing accuracy of a processing surface of the work 35, damaging the cutting tool 37, and the like. The cutting methods described in, for example, Japanese Registered Utility Model No. 2,590,593 and Japanese Unexamined Patent Publication No. S49-105277 were proposed to solve such problems. The method described in Japanese Registered Utility Model No. 2,590,593 suppresses vibration of the work 35 by bringing a steady rest into contact with the work 35 for lathe turning (cutting) of the work 35. The method described in Japanese Unexamined Patent Publication No. S49-105277 suppresses chatter vibration by periodically changing the rotational speed of the work 35, that is, the rotational speed of a main spindle.
However, the method described in Japanese Registered Utility Model No. 2,590,593 is not suitable for reducing the size of a machine tool because the steady rest for the work 35 is provided in a processing machine and thus a processing region is reduced. The method described in Japanese Unexamined Patent Publication No. 549-105277, on the other hand, changes the rotational speed of a main spindle during processing, and thus tends to leave marks on the processing surface of the work 35 due to the change in rotational speed, thereby causing a problem such as reduced finishing accuracy.
The chatter vibration and the dynamic rigidity of the work will now be described. The chatter vibration of the work is a vibration phenomenon which occurs during processing. Whether the chatter vibration occurs or not depends on the processing conditions, the dynamic rigidity of the work, and the like. The chatter vibration tends to occur under high-load conditions such as heavy cutting, and tends to occur when the work has low dynamic rigidity.
FIG. 12 shows an example of a dynamic compliance of the work which is an inverse number of the dynamic rigidity. This figure shows that the dynamic compliance is small (the dynamic rigidity is large) in the direction of 30°, and the dynamic compliance is large (the dynamic rigidity is small) in the direction of 120°. Thus, the dynamic compliance of the work is normally anisotropic, and the dynamic rigidity varies according to the angle. The same applies to the case where the work itself has a cylindrical shape and is isotropic. This is due to the influence of intrinsic anisotropy in the spindle stock.
This measurement result shows that the chatter vibration tends to be generated during cutting when the cutting angle toward the rotation center of the work is set to be 120° which is an angle with large dynamic compliance, while the chatter vibration is less likely to be generated and excellent cutting can be performed when the cutting angle is set to be 30° which is an angle with small dynamic compliance. Rotary cutting using a rotating tool as the cutting tool is similar in this regard. In rotary cutting as well, cutting at an angle with small dynamic compliance is equal to cutting at an angle with high dynamic rigidity, whereby accurate cutting can be achieved.